"This is probably Mahler's attempt to define the special kind of sonority he was aiming for by indicating that the bass instruments were to play pizzicato and arco simultaneously."
No, this is not what he meant at all. In the Leipzig edition of the first version, Mahler - always at the forefront of expanding the role and types of instruments in the Schlagzeug department (percussion) - wrote a part for a percussionist to drop stones into an overturned tenor drum, filled with water to the 7/8's full mark. After some dissatisfaction over this effect at the Prague premiere - which used the Graz edition but with the water drum/stones reinstated - Mahler brought his own stones from the Mondsee (Bad Ishil) to be used at the Amsterdam performance. However, both the Leipzig and Graz editions - thought to have been burned in a fire by Alma over her mistaken thought that the stones came from their second villa in Styria - have since totally vanished. There was also an incident where an inebriated percussionist at a Vienna Philharmonic rehearsal, overturned the tenor drum full of water and ruined some of the stage flooring at the Musikverein. The practice has since been completely dropped, and the plopping stones have since been delegated to the double basses. I know this to be true because Mengelberg's brother said so.